Electrical multilayer component and method for the production thereof

ABSTRACT

An electrical component includes a stack of layers. The layers include dielectric layers and electrode layers. The dielectric layers have a resistance with a positive temperature coefficient. The electrode layers are electrically conductive and are interspersed among the dielectric layers. At least one of the electrode layers includes a constituent that is comprised of a base metal and that is at least partially coated with a protective layer. The protective layer includes a material that slows oxidation of the base metal.

BACKGROUND

The invention concerns an electric multilayered component with a stackof layers, which include superimposed dielectric layers that cover aceramic material and electro-conductive electrode layers lying betweenthem. Beyond that, the invention concerns a method for producing themultilayered component.

In patent DE 197 19 174 A1, a component of the above type is known wherethe electric resistance of the dielectric layers exhibits a positivetemperature coefficient and where the electrode layers are produced fromaluminum. To produce such a multilayered PTC resistor, the use of basemetal for an electrode layer is necessary, because only such metalsform, for the purpose of a PTC element, a necessary oxide layer on theirsurfaces. This oxide layer is required for the barrier, which is a layerthat may be dismantled, between the electrode and dielectric layers.

The known component has the disadvantage that aluminum used for PTCresistor ceramic is not stable and oxidizes at typical sintertemperatures >1300° C. The electrode layers therefore exhibit a high ohmresistance due to the sinter that is undesirable for a multilayer PTCresistor.

Furthermore, the known PTC component has the disadvantage that aluminumis easily diffused into the ceramic at the highest sinter temperaturesof >1000° C. and impairs the desired characteristics of the PTC ceramic.

Moreover, the elements of the aforementioned method are known frompatent DE 199 16 380 A1 where the dielectric layers are selected from apiezoelectric material. The electrode layers are produced from a mixtureof silver and palladium

These known components have the disadvantage that the materials, silverand palladium, are expensive to obtain. More easily and cheaplyavailable materials, such as copper, require a very high,process-technical method in order to oxidize the copper.

SUMMARY

It is therefore the intent of the present invention to provide anelectric multilayered component that allows a usable metal to protectelectrode layers before they oxidize at high temperatures in oxygenatedatmospheres.

According to the invention, this goal is attainable with a multilayeredcomponent described in claim 1. Other forms of the invention and amethod for producing the invention are to be inferred from furtherclaims.

The invention is directed to an electric multilayered component thatincludes a stack of layers with superimposed dielectric layers. Thedielectric layers include a ceramic material and are arranged andseparated from one another by electro-conductive electrode layers lyingbetween them. At the least, an electrode layer includes a body coveredby a protective layer. The body includes a metal. The protective layerhas the task of hindering and slowing down oxidation of the body.Metals, particularly precious metals that exhibit a greater standardelectrode potential than the metal in the body, should be considered foruse in a protective layer. Another suitable compound such as boron- orsilicon-comprised glass, for example, could also comprise the protectivelayer.

Condensers, pyroelectric conductors, resistors and piezoelectriccomponents, in particular, are possibilities of multilayered components,according to the invention.

The multilayered component, according to the invention, has theadvantage that, due to the protective material included in it, theprotective layer shields the body from unwanted oxidation. Inparticular, the invention makes possible the use of base metals as themetal for the body. The base metals have the advantage that they arecheap and easily accessible. To clarify: the standard electrodepotential measured against a standard hydrogen electrode at 25° C. isnegative for all metals besides base metals.

Beyond that, the component according to the invention permits theapplication of process steps where the component is exposed during orafter the production of the oxygenated atmosphere. On account of theprotective layer, a multilayered component without a protective layerwhere the electrode layer is not completely oxidized can be used ratherthan either a higher partially pressurized oxygen or a highertemperature within the component or both. This is a particular advantageif a sinter process produces the multilayered component. Using basemetals in electrode layers, the partially pressurized oxygen, usuallyvery accurately reduced by air, must be regarded while sintering. Thecomponent, according to the invention, now makes it possible toaccomplish a sintering with relatively high partially pressurizedoxygen. This possibility simplifies the process as well as reduces itscosts.

Common sintering of ceramic green foils and electro-conductive layerscan produce a component, according to the invention, with particularadvantages. Such sintering will make it possible, in an easilycarried-out process, to stack many layers on top of one another and tocombine them, in one single step, into a monolithic component. Inparticular, the common sintering of ceramic green foils with electrodelayers allows the production of components with very many electrodelayers that can be used, for example, by condensers at a high capacity,by multilayered PTC resistors at a low resistance, and by piezoelectriccomponents at a high mechanical deflection.

Beyond that, in one implementation of advantage, outer electrodes, whichare in contact with the electrode layers, can be arranged along theouter surface of the component, according to the invention. Thisimplementation makes it possible to produce multilayered componentssuitable for surface montage technology. Suitable outer electrodes wouldbe, for example, electrodes arranged cap-like along two surfaces onopposite sides of the stack of layers. These electrodes can be easilysoldered with conductive strips on a conductive plate using surfacemontage technology.

An advantageous and broader implementation of the invention includescontact between neighboring electrode layers with different outerelectrodes. With this it is possible to arrange the electrode layers inthe form of interlinking comb structures. In particular, it can achievea high capacity via parallel circuitries for condensers with capacitiesin different parts, a reduced ground resistance via parallel circuitriesfor multilayered PTC resistors of partial resistance, and an increasedmechanical deflection for piezoelectric components.

In order to satisfy the requirements for specific sinter processes attemperatures >800° C., it is advantageous to select protective materialthat also slows down the oxidation of the body at sintertemperatures >800° C. With this it will be possible to create ceramicmultilayered components with base metals in the electrode layers thatwould become oxidized without a protective layer at the aforementionedsinter temperatures. Such ceramic components are, for example,piezo-actuators or multilayered PTC resistors.

While particular base metals could be considered for the metal of thebody, precious metals, which exhibit temperature characteristicssuitable for sintering in air, have been preferably used for theprotective material. In particular, silver, gold, platinum or palladiumshould be considered as the precious metals. However, other materialsare also possible, such as boron- or silicon-comprised compounds, forexample.

Suitable metals for the body are, in this case, tungsten, copper,nickel, chromium, aluminum or titanium. The metal, tungsten, is, in thiscase, also a suitable alternative for producing multilayered PTCresistors. Chromium and zinc also work as alternatives. Copper isparticularly suitable for producing piezoactuators, while the metal,nickel, together with a protective layer, according to the invention,proves favorable for condensers that can simplify processing on accountof possible sintering in air rather than sintering with reduced oxygencontent.

To create a ceramic multilayered PTC resistor, it is of particularadvantage if the ohm resistance of the dielectric layers exhibits apositive temperature coefficient. This is possible, for example, byusing PTC resistor ceramics. A suitable PTC resistor ceramic is, in thiscase, a barium titanate ceramic of a common compound (Ba, Ca, Sr, Pb)TiO3 that is comprised of donators and/or acceptors, in this case,manganese and yttrium.

By using such a ceramic, preferably for the barrier, a layer that may bedismantled, alternative base metals, such as aluminum, chromium or zinc,can be used in the electrode layers. However, tungsten, in particular,is also a suitable metal for the electrode layer. To clarify: beneaththe barrier, base metals have been oxidized at the electrode/ceramicborder, and the charge carrier concentration on an edge betweenelectrode and dielectric layers has been increased (accumulation=edge).A resistive contact has been constructed, which is necessary formultilayered PTC resistors to function. Without a protective layer, thenamed metals would, however, be completely oxidized during sintering inair. Such a layer is necessary for the simple creation of multilayeredcomponents for PTC resistor ceramics at typical sinter temperatures.Electrode layers and ceramic would, therefore, not be needed, becausethe oxidized electrode components diffuse out of the electrode/ceramicborder and into the ceramic. Sintering in an oxygenated atmosphere isnecessary in order to construct actively seeded borders of the PTCresistor ceramic, which are used for cooling after the sintering. Withhelp from the protective layer, however, necessary sinter conditions canbe carried out without completely oxidizing electrode layers oralternatives for electrodes.

Multilayered PTC resistors have been used for the purpose of protectingcomponents or modules from high currents. With a sudden rise in current,the resistance of multilayered PTC resistors increases very strongly,whereby a serially circuited component for the multilayered PTC resistoror the circuit, itself, can be effectively protected from a currentoverflow. After removing the flawed condition, which resulted from thehigher current, the multilayered PTC resistor cools off and returnsagain to a low ohm resistance. Ceramic multilayered PTC resistors havethe advantage, due to parallel circuitry of multiple single resistors,that they exhibit a very low partial resistance at low temperatures.Also, they always reliably achieve that low resistance again afterrepeated rises and falls of the current flowing through the PTCresistor.

With the implementation of the component, according to the invention,for multilayered PTC resistors, other materials, which comprise chemicalcompounds of tungsten, could particularly be considered for the body. Inparticular, tungsten carbide or tungsten nitride comes to mind. Tungstencompounds have the advantage that the oxidation of tungsten has beenslowed down or nearly prevented so that the necessary barrier, a layerthat may be dismantled, can still develop and is ensured despite a highconductivity within the electrode layer.

In one implementation of the invention, the electrode layers can includea body that may be comprised of layers. In each case, a protective layeris arranged on the body's top and bottom sides.

In a further implementation of the invention, the body, arranged intoelectrode layers, can also be made from a protective layer of particles.This implementation of the invention makes the use of powders possible.The powders include a multiplicity of such particles for producingelectrode layers by which the use of known (silk-) screen-printing hasbeen made possible. By using powder, an advantage that no newtechnologies can be developed for applying electrode layers to ceramicgreen foils and/or for their further processing results.

Beyond that, using coated particles in the electrode layer ofmultilayered PTC resistors has an advantage if the electrode layer nextto the particles still includes a precious metal like silver orpalladium so that a higher conductivity within the electrode layer hasalso been ensured for partial oxidation of the responsive electrodecomponents at the barrier, a layer that may be dismantled, in the coreof the particle. An electrode layer of a multilayered PTC resistor can,for example, be created with 10% (by weight) of coated tungsten powderand 90% (by weight) of a silver and palladium mixture.

Furthermore, the protective layer can exhibit at least two parts thatinclude different materials in the layer. For example, it comes to mindthat a powder should be used for the electrode layers, the particles ofwhich are essentially created from tungsten at their core, whereby asilver layer has coated the core of the particles. The silver-comprisedcoating is covered again by a second wrapping that includes palladium.One implementation of the protective layer, such as a double layer, hasthe advantage that during the heating process for sintering thecomponent, an alloy can form out of the silver and platinum. This alloymelts at a higher temperature than silver (silver melts at approximately900° C.) and can therefore eliminate the need for a partial barrier, alayer that may be dismantled. The protective layer thus does not allowtoo much oxygen to be admitted to the tungsten in the particle's core.

A suitable powder for use in the multilayered component, according tothe invention, can be produced using a physical method, for example,that coats particles of a suitable metal with a layer of a preciousmetal. Sputtering or evaporating should be considered for the physicalmethod to produce powder, the particles of which are coated. With this,however, that the particles of the powder must be moved during theevaporating or sputtering so that they have been coated on all sidesmust also be considered.

Boron- or silicon-comprised glass, used as the protective material, canbe crushed into a powder form of a protective layer by a chemicalprocedure such as PVD or CVD.

Using the multilayered component, according to the invention, as apiezo-actuator or as a condenser, has an advantage if the protectivelayer densely encloses the bodies. In these cases, oxidizing the body isnot desirable. By densely enclosing the body within the protectivelayer, the increase of oxygen, predicted from the oxygen transported bymeans of diffusion, can be decreased to a large extent.

With the named method for producing powder, the particles of which arecoated, protective layers develop that exhibit pores. As an advantage,these pores can allow oxygen to be admitted to the particle's core andtherefore service the barrier, a layer that may be dismantled, in themultilayered PTC resistors. On the other hand, however, in order toprevent the admission of too much oxygen to the body, it can beadvantageous to select thick layers of the protective layer that reducepore development and, as a result, reduce to an acceptable measurementthe admission of oxygen through the barrier, a layer that may bedismantled. A suitable thickness for the protective layer amounts to <5Tm with regard to an often used (silk-) screen-printing method.

Furthermore, there is an advantage if the particles, included within thepowder that has been used as a paste for producing the electrode layers,exhibit an expansion of typically <5 μm. Similarly, it is alsoadvantageous if the protective layer exhibits a thickness of typically<5 μm. Such particle dimensions have the advantage that the usual sievesfor (silk-) screen-printing can be used of laying electrode layers onceramic green foils. Average sieves typically have a mesh size smallerthan 5 μm.

However, other strong relationships between a particle and a protectivelayer are probable.

Furthermore, the invention concerns a method for producing an electricmultilayered component, whereby the sintering of layers has beenaccomplished at temperatures of typically above 800° C.

Such a method has the advantage that the necessary sinter temperaturefor many applications and/or ceramic materials can be achieved. Beyondthat, on account of the protective layer, according to the invention,inexpensive materials should be considered for the metal of the body.

Beyond that, a method for producing multilayered PTC resistors has anadvantage when the sintering of the stack of layers occurs in anatmosphere where the partially pressurized oxygen equilibrium of thebody metal/body metal oxide equilibrium is exceeded. Such a method hasthe advantage that with the oxygen pressure increased, in this case, byair that can occur during sintering, the process for producing thecomponent has been decidedly simplified. For example, this result canoccur from using tungsten as the metal for the body in an oxygenatedatmosphere that exceeds the partially pressurized tungsten/tungstenoxide equilibrium.

Thus, the necessary high-oxygen portion of the sinter atmosphere hasbeen supplied for many ceramics, whereby the protective layereffectively shields the base metal, in this case, tungsten, from thehigh partially pressurized oxygen.

The invention has been described on the basis of examples ofimplementations and, in the following, those are clarified by thefigures that apply to them.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in this case, a multilayered component, according to theinvention from a perspective view.

FIG. 2A shows the particles of a powder, which can be used for producingelectrode layers in the component, according to the invention, from aschematic cross-sectional view.

FIG. 2B shows a particle in accordance with FIG. 2A, the protectivelayer of which exhibits pores.

FIG. 3 shows, in this case, the structure of a component's electrodelayer, according to the invention, from a schematic cross-sectionalview.

DETAILED DESCRIPTION

FIG. 1 shows a component, from a perspective view, which has beenproduced by sintering of a stack of superimposed green foil andelectrode layers. On a surface of a green foil, an electrode paste hasadditionally been applied to the electrode in the indicated range.Additionally, a set of thick-layering methods, preferably imprinting,are, themselves, suitable, in this case, for (silk-) screen-printing. Asurface area not covered by electrode paste remains at least within therange of the green foil's edge or only within the range of the greenfoil's corner. It is also possible not to use the electrode in atwo-dimensional layer but instead in a structure and, if necessary, openpattern.

The (silk-) screen-printing paste exhibits, using metallic tungsten or atungsten compound, for example, extensive particles for producing thedesired conductivity and, if necessary, sinterable, ceramic particlesfor adjusting the weakened characteristics of the electrode paste tothat of the ceramic and an extinguishable, organic bond, in order toensure a malleability of the ceramic mass and/or an adhesion of thegreen bodies. Therefore, particles of pure tungsten, particles oftungsten alloys, tungsten compounds or mixed particles of tungsten, andother metals can be used. A protective layer, according to theinvention, therefore coats the particles (compare FIGS. 2A and B). Withceramic multilayered components, which only a small mechanical load hassuspended, it is also possible to do entirely without the electrodepaste for the ceramic portion.

Subsequently, the printed green foils have been superimposed, in adesired number, on a stack of foils where (green) ceramic layers andelectrode layers are arranged, alternating one on top of another.

Subsequently, the stack of layers, still elastic because of the bond,has been molded into the desired outer shape by pressing and, ifnecessary, by cutting. The ceramic is then sintered in what can be amulti-step process. The final sintering, which sinters together theceramics until completed and/or until the desired compression, occurs,as a rule, between 800 and 1500° C.

After sintering, a monolithic ceramic stack of layers 1 develops fromthe single layers of green foil, which exhibit a tight group ofdielectric layers 2 formed from single, ceramic layers. This tight groupis also found at the junction points between ceramic/electrode/ceramic.In the stack of layers 1, dielectric layers 2 and electrode layers 3 arearranged, alternating one on top of another. Outer electrodes 6 have nowbeen produced for two of the other sides, lying opposite each other, inthe component body. In each case, the outer electrodes standelectrically in contact with each second electrode layer 3.Additionally, in this case, a ceramic metallurgy can be produced usuallyfrom silver, in this example, by a current separation. This cansubsequently be strengthened exceedingly, for example, by using an orderof layers Ag/Ni/Sn. The soldering ability for platinum has thereforebeen improved. However, other possibilities are suitable for makingelectrode layers 6 into metal and/or producing them.

The following advantages have been achieved using coated tungstenparticles in accordance with FIG. 2, including electrode layersconnected with a PTC resistor ceramic.

-   -   a. minimization of oxidation, therefore minimized expansion of        volume    -   b. improvement of ceramic's adhesive strength    -   c. improvement of electric conductivity resulting from less        oxidation    -   d. better ability to make compounds from a paste made of silver        that can be used for engraving on electrode layers; the        compounds can be used for outer metallurgy    -   e. load distribution within the layer has been comparably        moderated due to an improved homogeneity resulting from less        oxidation    -   f. the barrier, a layer that may be dismantled, has been made        using tungsten in combination with PTC resistor ceramic,        producing a resistive contact

The invention does not limit itself to PTC resistor ceramics withtungsten-comprised electrode layers, but instead is applicable to manyother types of electro-ceramic components, such as, in this case,condensers or piezo-components, which may preferably find use inperowskit ceramic or in heat conduction associated with spinel ceramic.Beyond that, such components, according to the invention, should beconsidered with which the ceramic layers include a zinc oxide ceramicand are therefore suitable as resistors. Beyond that, the component,according to the invention, can be used as, in this case, a PTC resistorwith dielectric layers, barium titanate ceramic with the additions:barium, titanium, calcium, strontium or lead and/or further remunerativeelements.

FIG. 2A shows a body 4 in the form of a particle, which is coated by aprotective layer 5 that densely encloses the particle. In order toachieve the necessary partial oxidation for use in multilayered PTCresistors, porous protective layers can be produced and/or the densepart of the protective layer can be adapted so to allow the admission ofsmall amounts of oxygen into the body. FIG. 2B shows such a particle,the protective layer 5 of which contains pores 7. The body 4 can, inthis case, be created from tungsten, while the coating is created frompalladium as the protective material.

FIG. 3 shows an electrode layer 3, with which the body 4 exhibits theform of a layer that is covered by a protective layer 5 along the topand undersides. The protective layer 5 can, in this case, be createdfrom palladium, while the body 4 can include tungsten.

The invention does not limit itself to the described examples ofimplementations but instead has been defined in its most general form byclaim 1.

1. An electrical component comprising: a stack of layers, the layerscomprising: dielectric layers comprised of a ceramic material; andelectrode layers between at least some of the dielectric layers; whereinat least one of the electrode layers comprises a body that has top andbottom sides that are covered by a protective layer, the body comprisingmetal, and the protective layer comprising a protective material thatslows oxidation of the metal.
 2. The electrical component of claim 1,wherein the dielectric layers comprise ceramic green foils that aresintered jointly with the electrode layers.
 3. The electrical componentof claim 1, wherein the protective material slows oxidation of the metalduring sintering at temperatures >800° C.
 4. The electrical component ofclaim 1, wherein the metal comprises a base metal.
 5. The electricalcomponent of claim 1, wherein the protective material comprises aprecious metal.
 6. The electrical component of claim 1, wherein themetal comprises at least one of tungsten, copper, nickel, aluminum,titanium and chromium.
 7. The electrical component of claim 1, whereinthe protective material comprises at least one of gold, silver, platinumand palladium.
 8. The electrical component of claim 1, wherein aresistance of the dielectric layers has a positive temperaturecoefficient.
 9. The electrical component of claim 1, wherein theelectrode layers include at least one of tungsten carbide and tungstennitride.
 10. The electrical component of claim 1, wherein the protectivelayer is comprised of a plurality of materials.
 11. The electricalcomponent of claim 1, wherein the electrode layers are produced from apowder comprised of particles, and particles of the at least oneelectrode layer are coated with the protective material via a chemicalprocess or a physical process.
 12. The electrical component of claim 1,wherein the protective layer restricts exposure of the body to oxygen.13. The electrical component of claim 1, wherein the protective layer isporous.
 14. The electrical component of claim 1, wherein the electrodelayers are produced from a powder comprised of particles, particles ofthe at least one electrode layer have a maximum expansion of 5 μm, andthe protective layer has a maximum thickness of 5 μm.
 15. The electricalcomponent of claim 1, further comprising: outer electrodes that contactthe electrode layers, the outer electrodes being on outer surfaces ofthe stack of layers.
 16. The electrical component of claim 15, whereinadjacent electrode layers contact different outer electrodes.
 17. Amethod of producing an electrical component comprised of a stack oflayers, the stack of layers comprising dielectric layers comprised of aceramic material and electrode layers between at least some of thedielectric layers, at least one of the electrode layers comprising abody that has top and bottom sides that are covered by a protectivelayer, the body comprising metal and the protective layer comprising aprotective material that slows oxidation of the metal, the methodcomprising: sintering the dielectric layers and the electrode layers attemperature > 800 °C, the dielectric layers comprising green foil. 18.The method of claim 17, wherein sintering is performed in an oxygenatedatmosphere where an oxygen partial pressure of the atmosphere exceeds anequilibrium oxygen partial pressure of metal/metal oxide in the body ofthe electrode.